But research from University College London, Queen Mary London and Cancer Research UK suggests the guidance has been largely ignored.

Their study, involving almost 1,000 GPs, found just 24 per cent were aware of the guidance.

Almost half of family doctors did not even know that the drug reduces the chance of breast cancer.

Tamoxifen works by blocking out the cancer-triggering hormone oestrogen in the breast cells. This stops breast cancer cells growing in the first place, or returning in women who have already had the illness.

Patients are advised to take the drug once a day for five years, with trials showing that the protection can last for two decades.

Each year there are more than 53,000 new cases of breast cancer in the UK and the illness causes 11,000 deaths.

The average woman has a one in eight chance of developing it in her lifetime, rising to one in three for those at high risk.

Baroness Delyth Morgan, chief executive at Breast Cancer Now, said: “It is extremely concerning that many women at an increased risk of breast cancer are still not being offered the choice of taking tamoxifen to reduce their risk.

“Nice’s 2013 guideline recommended that it be offered to all women at medium and high risk of the disease. But unfortunately this is largely not being adhered to, with many GPs lacking confidence in discussing the option with patients and a worrying number not even being aware of it.”

Tamoxifen is only licensed for treatment of breast cancer. The guidance from 2013 advises doctors also to prescribe it to prevent disease, but the lack of a licence has left many GPs confused, the research suggests.

Prof Arnie Purushotham, Cancer Research UK’s senior clinical adviser, said: “Cancer-preventing drugs have the potential to have a huge impact.”

The study found doctors were far more likely to be aware that aspirin could reduce the risk of bowel cancer.